Osmotic pressure is produced when solutes in aqueous solution at one side of an osmotic reverse osmosis medium are at a chemical potential less than that in pure aqueous solvent (water) located on the other side of the reverse osmosis medium and as a result the solvent naturally permeates through the reverse osmosis medium into the aqueous solution to equalize the chemical potentials thereby exerting an osmotic pressure across the reverse osmosis medium.
In reverse osmosis, pressures greater than the osmotic pressure are exerted on the aqueous solution and this has the effect of inducing the solvent to permeate through the reverse osmosis medium from solution, while the solute is excluded from diffusion and remains behind in solution. Therefore the aqueous solution becomes more concentrated, and water with a low solute concentration accumulates on the other side of the reverse osmosis medium. By this process water can become “desalinated”.
Reverse osmosis has become a much more feasible technology for the desalination of non-potable water supplies, and although still more expensive than conventional water extraction the technology is starting to be used in more remote regions, arid areas where there are few alternatives. Examples of the latter are the oil-rich Gulf states, arid regions in California and elsewhere in the US. In Australia, the Ayers Rock resort at Yulara relies on RO-treated groundwater. Additionally, Perth in WA is currently commissioning a large seawater desalination plant as a preferred option to address increased demand and reduced rainfall.
Desalination by reverse osmosis is energy-expensive generally, as more saline, non-potable water is pumped under pressure through a reverse osmosis medium separation chamber and aqueous solvent diffuses through the reverse osmosis medium whilst salts and contaminants are rejected by the reverse osmosis medium and concentrate in the feed stream. This gives rise to a saline waste which requires proper disposal.
In addition in existing reverse osmosis systems these require facilities for the above-ground storage of the water which is to be desalinated in the reverse osmosis device. This exposes the water to oxygen and sunlight which results in increased biological activity such as algal growth and physicochemical reactions such as iron oxidation and precipitation which can cause reverse osmosis medium clogging, thus water must be pretreated before desalination.